Digital capacitive membrane transducer

ABSTRACT

A capacitive membrane is used as a digital sensor. Membranes act as binary devices, such as being in a collapsed or non-collapsed state. By providing drum heads (membranes and associated gaps) with different response characteristics, the drum heads of an element digitally indicate the amplitude of the acoustic force by which of the drum heads are triggered or change states. The digital transducer may be used for different types of sensors, such as a CMUT, an air pressure, a temperature, a humidity, a chemical or biological stimulus or other sensor.

BACKGROUND

This present description relates to capacitive membrane transducers. Forexample, capacitive membrane ultrasound transducers (CMUT) are provided.

CMUTs provide a greater bandwidth than piezoelectric-ceramictransducers. An array of elements, such as a two-dimensional array ofelements is formed using microelectromechanical processes. Each elementincludes a plurality of membranes with associated electrodes separatedby a gap or void, which transduce between electrical and acousticenergy. Flexing of the membranes in response to acoustic energygenerates an analog electrical signal representing the amount offlexing. However, CMUTs typically have a large impedance mismatch withthe receive beamformer electronics due to the low reactance of the CMUT.Receiving an analog signal with a desired dynamic range may use complex,expensive, or large circuits to interface between the CMUT and thereceive beamformer. The interface electronics may also requireco-location with the CMUT elements, adding to the size, weight, and heatload of the transducer head.

BRIEF SUMMARY

By way of introduction, the preferred embodiments described belowinclude methods, systems and sensors for a digital capacitive membranetransducer. The capacitive membrane is used as a digital sensor. Forexample, the membranes are binary devices, such as being in a collapsedor non-collapsed state. By providing drum heads (membranes andassociated gaps) with different response characteristics, the drum headsof an element digitally indicate an amplitude of the acoustic energy bywhich of the drum heads are triggered or change binary states. Thedigital transducer may be used for different types of sensors, such as aCMUT, an air pressure, a temperature, a humidity, a biological stimulusor other sensor.

In a first aspect, a method is provided for receiving acoustic energywith a capacitive membrane ultrasound transducer. A first membraneoperates as a first binary acoustic sensor. An output of a first elementof the capacitive membrane ultrasound transducer is determined as afunction of the first binary acoustic sensor.

In a second aspect, a system is provided for receiving ultrasoundenergy. A first element has a plurality of acoustic drum heads. At leasttwo of the acoustic drum heads have different acoustic responsecharacteristics. An encoder connects with the first element. The encoderoutputs digital information as a function of collapse, snap-back or bothcollapse and snap-back operation of the acoustic drum heads.

In a third aspect, a sensor is provided for detecting a characteristic.A plurality of capacitive membrane transducers has hysteretic bistablestates. An encoder measures the characteristic as a function of outputsof the capacitive membrane transducers.

In a fourth aspect, a method is provided for receiving acoustic energywith a capacitive membrane ultrasound transducer. A first membraneoperates as an acoustic sensor with more than two digital states. Forexample, the collapsed state may be differentiated into several discretesteps, via separate electrodes. Any of several output levels of a firstelement of the capacitive membrane ultrasound transducer are determinedas a function of the first acoustic sensor.

The present invention is defined by the following claims, and nothing inthis section should be taken as a limitation on those claims. Furtheraspects and advantages of the invention are discussed below inconjunction with the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The components and the figures are not necessarily to scale, emphasisinstead being placed upon illustrating the principles of the invention.Moreover, in the figures, like reference numerals designatecorresponding parts throughout the different views.

FIG. 1 is a block diagram representation of one embodiment of a sensor;

FIG. 2 is a cross-sectional view of one embodiment of membranetransducer in an non-collapsed state;

FIG. 3 is a cross-sectional view of the membrane transducer of FIG. 2 ina collapsed state;

FIG. 4 is a block diagram representation of one embodiment of a CMUT;and

FIG. 5 is a flow chart diagram of one embodiment of a method for sensinga characteristic with a digital sensor.

DETAILED DESCRIPTION OF THE DRAWINGS AND PRESENTLY PREFERRED EMBODIMENTS

Different membranes each operate in a collapse and snap-back mode inresponse to different influences. One or more bits of information areprovided from each of the different membranes. For a CMUT, each elementincludes the different membranes. The membranes collapse or snap back atdifferent acoustic pressures. The different membranes are responsive todifferent pressures or rarefactions within a desired range, such asabout 30 dB of dynamic range. By determining which membranes collapse orsnap-back, the pressure or rarefaction is measured using binary or otherdigital sensors. Since no analog signal is processed, the receivesignals output for an element may be less susceptible to noise, possiblyallowing transmission to a remote receive beamformer withoutamplification.

FIG. 1 shows a system for receiving a signal, such as ultrasound energy.The system includes a substrate 12, one or more membranes 14, a sensorsection 16, an encoder 18, electrical connections 20 and an output 22.Additional, different or fewer components may be provided. In oneembodiment, the system is part of a CMUT where the sensor section 16 isan element of an array. The system is a different type of sensor inother embodiments.

The membranes 14 are part of drum heads 26, such as acoustic drum heads26 for a CMUT. FIGS. 2 and 3 show example embodiments of an acousticdrum head 26. The acoustic drum head 26 includes the membrane 14positioned over an air, gas, vacuum, or liquid filled gap 28. The gap 28is thin, such as about 0.2 to 0.005 micrometers. Thicker or thinner gaps28 may be provided. The membrane 14 is thin, such as about 1 to 0.01micrometers, but thicker or thinner membranes 14 may be used. A pair ofelectrodes 32, 34 is provided on different sides of the gap 28, such asone electrode 32 in a bottom of the gap 28 and another electrode 34 on atop of the membrane 14. Different electrode positions may be used. Aninsulator 30 is provided within the gap 28, such as on the electrode 32as shown, on the membrane 14 or both. Additional, different or fewercomponents may be provided, such as the drum head 26 being free of theinsulator 30, or being made of electrically conducting material thatacts as a common ground.

The drum heads 26 are formed using microelectromechanical processes,such as semiconductor manufacturing processes. Using CMOS, deposition,sputtering, patterning, etching or other techniques, the variouscomponents are formed, including electrical connections 20, on or in thesubstrate 12. The substrate 12 is a semiconductor, such as silicon, orother now known or later developed material for forming the drum head26.

The insulator 30 is a semiconductor insulator, such as silicon oxide.Alternatively, the insulator 30 is a high-permittivity insulator, suchas titanate or sapphire based insulators. High-permittivity orhigh-dielectric insulators may have a piezoelectric property, such asconverting pressure caused by collapse of the membrane 14 as shown inFIG. 3 into electrical energy. The electrical energy is used with thecapacitive effects of collapse to more readily detect the collapse. Theinsulator 30 is thin, such as about 50 nm, but thicker or thinnerinsulators 30 may be used. In one embodiment, the insulator 30 has athickness of 0.4 micrometers and a relative dielectric constant of about10. If the relative permittivity is higher, such as 100, the insulatorthickness may be larger and still yield the same or similar receivesensitivity, which may be desirable depending on the voltages ofoperation.

Referring to FIG. 1, the sensor section 16 (e.g., an element of a CMUT)includes a plurality of membranes 14 and associated drum heads 26. Forexample, at least fifty, one hundred, two hundred and fifty six or othernumber of membranes 14 and associated drum heads 26 are provided for asingle acoustic element. The membranes 14 are distributed in any desiredpattern. In alternative embodiments, a single membrane 14 is provided.

The drum heads 26 have different response characteristics, such asdifferent ones or groups of the drum heads 26 responding differently todifferent acoustic amplitudes, pressures or powers. Different responsecharacteristics are provided by different diameters, membranethicknesses, gap depths or combinations thereof. In one embodiment, thediameter or lateral extent of the gap 28 and associated membrane 14 varyto provide different response characteristics. The thickness issubstantially uniformly thin or also varies. Similarly, the gap depth issubstantially uniformly thin or also varies. For example, one or moredrum heads 26 have a vacuum gap 28 of 0.01 micrometers, a membranethickness of 0.2 micrometers and a membrane radius of about 15micrometers. One or more other drum heads 26 have a different membraneradius, such as 15.2 micrometers. The difference in membrane radius orother characteristic results in different operation.

A drum head may make reversible contact with more than one electrode inthe course of collapse and snap-back. In one embodiment, the floor ofthe gap contains multiple electrodes, such as in the form of concentricrings, which yield a multi-bit stepwise response to varying acousticpressure and/or rarefaction, as the number of electrodes contacted bythe membrane changes.

Different response characteristics are provided additionally oralternatively by operating some drum heads 26 in a normally open modeand others in a normally closed (collapsed) mode. The drum heads 26 arebiased without application of the sensed energy in one of the binarystates. FIG. 2 shows the drum head 26 for a normally open mode whereacoustic pressure causes the membrane 14 to collapse in the gap 28.Positive pressure is sensed by collapsing the membrane 14. The lesseningor release of positive pressure is sensed by the membrane 14 returningor snapping back to a position above the gap 28. FIG. 3 shows the drumhead 26 for a normally collapsed mode where acoustic rarefaction orsuction causes the membrane 14 to extend from the bottom of the gap 28.The lessening or release of suction causes the membrane 14 to snap backto a collapsed position.

The drum heads 26 collapse and snap back at different voltages,pressures or other signal inputs. In one embodiment, the collapse andsnap-back have a hysteretic character. For example, collapse of one ormore membranes 14 occurs at 10 volts or −15 dB acoustic pressure leveland snap-back occurs at 8 volts or −20 dB. Other absolute or relativevalues may be used, such as where different membranes 14 or drum heads26 have different characteristics. The hysteresis may be made larger orsmaller, such as approaching zero through the use of small gaps and lowvoltage. By using lower voltage operation, the amount of acousticpressure or other outside power required to modulate between thecollapses and normal deflected or snap-back states may be lessened. Thehysteretic bistable state is based on exposure to chemical species ingaseous state, liquid state, solid state, plasma state or combinationsthereof.

The bias voltage applied to the membranes 14 may affect the hysteresis.Different bias voltages at different times and/or to different membranes14 are used to alter the hysteretic or response characteristic of thedrum heads 26. Varying the bias voltage may provide for a greaterresolution within a given dynamic range. For example, a given membrane14 is operated to trigger or alter states at different powers. In anacoustic example, intermediate collapse pressures are provided byramping or changing the bias voltage over the course of several transmitcycles. The variation alters the collapse and/or snap-back voltages,tuning the collapse and snap-back pressures slightly. By receiving inresponse to different transmit pulses with different bias voltages, asame drum head 26 triggers or detects different acoustic energy levels.Voltage ramping alone may be used without variation in the responsecharacteristic of the drum heads 26. However, at higher voltages, thecollapse-snapback hysteresis widens, allowing less data to be recorded.

The encoder 18 connects with the element 16. The connections areelectrical connections. Separate electrical connections 20 are providedfor each of the drum heads 26, but one or more drum heads 26 may sharean electrical connection with the encoder 18. A single encoder 18 isprovided for each element 16 as shown in FIG. 4. Alternatively, multipleencoders 18 are provided for each element 16 or the drum heads 26 ofmultiple elements 16 connect with a same encoder 18.

The encoder 18 and drum heads 26 are on a common substrate 12. Theelectrical connections 20 may be smaller or formed as part of anintegrated circuit on the common substrate 12. Given the digital orbinary operation of the drum heads 26 and detection of the state of thedrum heads 26, an amplifier between the drum heads 26 and the encoder orbetween the drum heads 26 and any receive beamforming circuitry may beavoided. For example, the common substrate 12 with the element 16 andthe encoder 18 are free of an amplifier. Alternatively, the encoder 18and drum heads 16 are on separate substrates.

The encoder 18 is a processor, digital signal processor, applicationspecific integrated circuit, field programmable gate array, analogcircuit, digital circuit, integrated circuit, combinations thereof orother now known or later developed circuit. In one embodiment, theencoder 18 contains an impedance analyzer, such as a voltage or currentdetector.

The encoder 18 outputs digital information as a function of collapse,snap-back or both collapse and snap-back operation of the acoustic drumheads 26. The encoder 18 operates with or includes a clock or clocksignal, such as a clock and signal shared by all or multiple encoders18. The encoder 18 measures a state of each of the drum heads 26periodically, such as every millisecond. By determining which drum heads26 have or have not altered states at a give time, the encoder 18determines the force applied to the element 16. For example, one half ofthe drum heads 26 operate in a normally closed mode and do not alterstate in response to positive pressure. The other half of the drum heads26 operate in a normally open mode. The positive pressure is sufficientto collapse some or all of the drum heads 26. As the positive pressureincreases or decreases, different numbers or ones of the drum heads 26change state. The encoder 18 measures a current positive pressure bydetermining which of the normally closed drum heads 26 have opened, andwhich of the normally open ones have closed. The measurement andassociated time of measurement are output as digital information. Overtime, the outputs represent the detected pressure as a function of time.Where the received signal varies over time, such as in periodicultrasound signals, the different amplitude time-steps digitallyrepresent the analog force applied to the element 16.

The encoder 18 or a receive beamformer reassembles or uses the measuredvalues to determine samples representing scanned locations at differenttimes. Frequency or time domain analysis may be used. The differentamplitude steps are assembled at their respective delays into aquasi-analog response in the frequency domain, or the bit streams areprocessed in the time domain via their autocorrelation functions. Theautocorrelation function is defined as the autocovariance divided by thevariance, and, for a time sequence, represents the time required for asignal to become random, i.e., uncorrelated. The time autocorrelationfunction is the time-frequency transform of the spectral densityfunction, containing the same information as the power spectrum. Thetime autocorrelation function best suited for acoustic analysis is ascaled type, in which multiple states or degrees of correlation aredetected at each time interval, but clipped or other functions may beused.

The output digital information is a first signal representing receivedpower, such as acoustic power, for the entire element 16. The binaryreadings (e.g., collapsed or not collapsed) of the drum heads 26 isdigital. The signal representing the received power is digital. Theoutput digital information is a value representing an acoustic pressureor other power as a function of binary readings from the plurality ofacoustic drum heads 26. The encoder 18 and element 16 operate as adigital sensor, such as digital CMUT, reducing susceptibility to noiseand decreasing the necessity for power-consuming and heat-generatingamplifiers.

FIG. 4 shows an array of elements 16 and associated encoders 18. Forexample, the array of elements 16 acts as an ultrasound transducer arrayfor receiving acoustic echoes. Any number of elements 16 may be used,such as 128, 192, or 256. The array is linear, curved, one dimensional,multi-dimensional (e.g., 1.5 D) or two dimensional. Each element 16includes a plurality of drum heads 26 for receiving energy. The encoders18 measure the amplitude of received pressure at different times bycounting the numbers of open and collapsed drum heads. A receivebeamformer connects with the encoders 18, such as on the commonsubstrate 12, on a different substrate and/or through one or morecables. The trace or electrical connection 22 (see FIG. 1) outputs themeasurements for beamforming.

FIG. 5 shows a method for receiving acoustic or other energy with acapacitive membrane ultrasound or other transducer. The method mayinclude additional, different or fewer acts, such as including atransmit act. The acts are performed in the order shown or a differentorder.

In act 50, an array of digital sensors, such as binary or three statesensors, is provided. Alternatively, a single digital sensor, such assingle drum head 26 or an element 16 with a plurality of drum heads 26,is provided. The digital sensors are the sensors described above forFIGS. 1, 2, 3 and/or 4, or other now known or later developed digitalsensors.

In act 52, the sensors are operated. Membranes of the different sensors,such as a binary acoustic sensor, are operated automatically in responseto applied power, such as acoustic energy or pressure. By operating thesensors as digital in function, such as collapsed and non-collapsedstates, analog signals showing variation in membrane position areavoided. By using sensors with different response characteristics, suchas membranes with different diameters, the sensors alter states inresponse to different applied pressures.

The membranes operate in a normally closed or collapsed state, anormally open state, or both. For example, a first set of the digitalstate sensors (e.g., membranes) operate in a normally open mode. Themembrane is normally not collapsed, so a change in the binary state isdetected in response to membrane collapse triggered by applied pressure.A second set of the state sensors operate in a normally closed mode. Themembrane is normally in a collapsed position, so a change in the binarystate is detected in response to membrane snap-back or movement to anon-collapsed state, which is triggered by applied rarefaction. Thetransition back to a normal state may also be detected. Where thetransitions away from normal and back to the normal state are associatedwith different voltages, capacitances, or powers, the binary statesensors are hysteretic bistable devices. Other sensors may incorporatedetection of multiple collapsed states, corresponding to differentapplied levels of pressure or rarefaction, in which only thelowest-force step, between the open and collapsed states, displayshysteresis.

In act 54, values are determined from the sensors, such as from an arrayof sensors. For an acoustic example, an output of each element of thecapacitive membrane ultrasound transducer is determined as a function ofthe digital acoustic sensors. An output is provided for each element inthe acoustic array. Each element may include one or a plurality ofdigital sensors. By measuring the state or membrane position of all ofthe sensors that comprise one element, the acoustic amplitude isdetermined.

As an alternative or in addition to measuring an amplitude with thedigital sensors, a time of a change in a state of the digital acousticsensor is measured. The change indicates a pressure or energy associatedwith the change of the sensor. More simply, the sensor may be used as adetector for a threshold for sufficient pressure. The time of triggeringor alteration indicates the existence of an echo. The time is usedwithout an indication of amplitude for generating an image or detectingan event of interest. For example, a dielectric insulator layer such asa ferroelectric oxide located between the membrane and the cavity floormay be used to generate a voltage pulse at the instant of collapse orsnapback, transducing the mechanical energy of membrane motion into avoltage spike that is readily detectable and localizable in time.

The binary state sensors are provided for detecting a characteristic. Aplurality of capacitive membrane transducers have hysteretic bistablestates, providing for sensing two different values of a characteristicwith a same sensor. By distributing a plurality of capacitive membranetransducers in multiple elements, the characteristic is sensed as afunction of location.

The capacitive membrane transducers or other sensor structures areoperable as binary sensors. The sensors change state as a function ofthe magnitude of the characteristic. Different ones of the capacitivemembrane transducers respond to different magnitudes of thecharacteristic.

The capacitive membrane transducers or other sensor structures areoperable as multistep digital sensors. The sensor changes state as afunction of the magnitude of the characteristic, with more than twoseparate states detected through a multiplicity of electrodes, each ofwhich yields a large, nonlinear change in impedance as the membraneconnects to or disconnects from it. Different electrodes within thecapacitive membrane transducer respond to different magnitudes of thecharacteristic.

Any one or more of many different characteristics may be sensed. Forexample, the hysteretic bistable states are responsive to ultrasound,temperature, air pressure, biological stimulus, or humidity. Fortemperature, the membranes, beams or other structures of the binarysensors are temperature sensitive. Bi-layer metals or other materialsrespond to temperature. For humidity, organic materials may be used inthe sensor for reacting to humidity. Different holes in membranes, otherstructures or different materials allow for different response ofdifferent ones of the binary sensors. Different materials used fordifferent binary sensors vary the response characteristic to biologicalstimulus.

An encoder measures the characteristic as a function of outputs of thecapacitive membrane transducers or binary state sensors. Separatelymeasurements are provided for each of multiple elements for scanning orsteering. Alternatively, multiple elements are provided for redundancy.Single element sensors may also be used.

While the invention has been described above by reference to variousembodiments, it should be understood that many changes and modificationscould be made without departing from the scope of the invention. It istherefore intended that the foregoing detailed description be regardedas illustrative rather than limiting, and that it be understood that itis the following claims, including all equivalents, that are intended todefine the spirit and scope of this invention.

1. A method for receiving an acoustic signal with a capacitive membraneultrasound transducer, the method comprising: operating a first membraneas a first digital acoustic sensor; and determining an output of a firstelement of the capacitive membrane ultrasound transducer as a functionof the first digital acoustic sensor.
 2. The method of claim 1 whereinoperating comprises operating the first membrane and a plurality ofadditional membranes as digital acoustic sensors, including the firstdigital acoustic sensor, the first membrane and the plurality ofadditional membranes having different diameters, and wherein determiningcomprises determining the output as a function of the digital acousticsensors of the first membrane and the plurality of additional membranes.3. The method of claim 1 further comprising: operating a second membraneas a second digital acoustic sensor; and determining an output of asecond element of the capacitive membrane ultrasound transducer as afunction of the second digital acoustic sensor.
 4. The method of claim 1wherein determining the output is performed without prior amplificationof signals from the first digital acoustic sensor.
 5. The method ofclaim 1 wherein operating comprises operating the first membrane as ahysteretic bistable device.
 6. The method of claim 1 wherein operatingcomprises operating the first membrane in a normally closed, normallyopen, or both normally closed and open states.
 7. The method of claim 2wherein determining the output comprises determining an acousticamplitude as a function of binary, ternary, or other states of thedigital acoustic sensors.
 8. The method of claim 1 wherein determiningthe output comprises determining a change in a state of the firstdigital acoustic sensor as a function of time.
 9. The method of claim 2wherein operating comprises operating a first set of the digital statesensors in a collapsed mode and operating a second set of the binarystate sensors in a normally open mode, the collapsed mode correspondingto changing a binary state in response to membrane opening, and the openmode corresponding to changing the binary state in response to membranecollapse.
 10. A system for receiving ultrasound energy, the systemcomprising: a first element having a plurality of acoustic drum heads,at least two of the acoustic drum heads having different acousticresponse characteristics; an encoder connected with the first element,the encoder operable to output digital information as a function ofcollapse, opening, or both collapse and opening operation of theacoustic drum heads.
 11. The system of claim 10 wherein the differentacoustic response characteristics comprise different diameters, membranethicknesses, gap depths or combinations thereof.
 12. The system of claim10 wherein the acoustic drum heads comprise multiple concentricelectrodes.
 13. The system of claim 10 wherein the first element has atleast fifty acoustic drum heads, the fifty acoustic drum heads eachhaving a unique acoustic response characteristic.
 14. The system ofclaim 10 wherein a first set of the plurality of acoustic drum heads isoperable in a collapsed mode and a second set of the plurality ofacoustic drum heads is operable in a normally non-collapsed mode. 15.The system of claim 10 wherein the acoustic drum heads separatelyelectrically connect with the encoder, the output digital informationbeing a first signal representing received acoustic power for the entirefirst element.
 16. The system of claim 10 wherein the first element andthe encoder are on a common substrate.
 17. The system of claim 16wherein the common substrate is free of an amplifier.
 18. The system ofclaim 10 wherein each acoustic drum head comprises a membrane with acorresponding gap and an insulator within the gap, the insulator havingpiezoelectric properties.
 19. The system of claim 10 wherein the outputdigital information is a value representing an acoustic pressure as afunction of binary, ternary, or other digital readings from theplurality of acoustic drum heads.
 20. The system of claim 10 furthercomprising: a second element having another plurality of acoustic drumheads; and another encoder connected with the second element, the otherencoder operable to output digital information as a function ofcollapse, opening or both collapse and opening operation of the acousticdrum heads of the second element.
 21. A sensor for detecting acharacteristic, the sensor comprising: a plurality of capacitivemembrane transducers having hysteretic bistable states; and an encoderoperable to measure the characteristic as a function of outputs of thecapacitive membrane transducers.
 22. The sensor of claim 21 wherein thehysteretic bistable states are responsive to ultrasound, temperature,air pressure, biological stimulus, or humidity.
 23. The sensor of claim21 wherein the hysteretic bistable states are responsive to exposure tochemical species in a gaseous state, a liquid state, a solid state, aplasma state, or combinations thereof.
 24. The sensor of claim 21wherein the capacitive membrane transducers are operable as binary,ternary, or other digital sensors as a function of the characteristic,different ones of the capacitive membrane transducers responsive todifferent magnitudes of the characteristic.
 25. The sensor of claim 21wherein the plurality of capacitive membrane transducers are distributedin at least two elements, the encoder being operable to measureseparately for each of the at least two elements.
 26. The sensor ofclaim 21 wherein the hysteretic bistable states are in a collapsed stateand a non-collapsed state.
 27. The sensor of claim 26 wherein a first ofthe capacitive membrane transducers is biased to a collapsed state and asecond of the capacitive membrane transducers is biased to anon-collapsed state.
 28. A method for receiving acoustic energy with acapacitive membrane ultrasound transducer, the method comprising:operating a membrane as an acoustic sensor with more than two digitalstates; and determining one of several output levels of an element ofthe capacitive membrane ultrasound transducer as a function of theacoustic sensor.
 29. The method of claim 28 wherein operating comprisesdetecting a plurality of different digital states during collapse of themembrane.